How to quickly fix (or avoid) the n-gons caused by boolean operation?
Select the N-gons with Shift+G
CTRL+T
Alt+J


This is the fast and easy way. The minus is that it's not always all quads. And avoid Boolean if you can as corrections are required if you use it

OR, easier yet



Use CTRL+R once on the pointed edge and snap it to vertices before you OK it.

Btw. okaying answers gives you +2 reputation.